1. Field of the Invention
This invention relates to a color sensor having at least one color filter, and more particularly to improvements in the sensitivity of the color sensors and minimization of variation of the spectral sensitivity within single sensors and among these sensors.
2. Description of the Prior Art
A prior art color sensor is disclosed, for example, in U.K. Pat. No. 2,115,980. In such a color sensor, color filters for selectively transmitting their respective monochromatic lights such as red, green, blue, etc. with the respective local wave length ranges are disposed on a main surface of a colorless and transparent insulator substrate such as a glass plate. To detect light transmitted through each of the color filters, a transparent front electrode layer, a semiconductor layer for photoelectric effect and a back electrode layer are stacked in order on the opposite main surface of the transparent substrate, corresponding to each of the filters. Generally, the front electrode comprises a single layer or stack of layers of transparent conductive oxides (TCO) such as indium tin oxide (ITO) and tin oxide (SnO.sub.x), and may be deposited by a vacuum evaporation method, an electron beam evaporation method, a sputtering method, a CVD method, a spray method or the like.
The refractive index of such a TCO layer is generally about 2.0, while the index of a semiconductor layer which is to be formed on the TCO layer is generally much larger than 2.0. For example, the index is about 4.0 for amorphous silicon derivatives such as amorphous silicon, amorphous silicon carbide and amorphous silicon germanium. Therefore, incoming light is partly reflected due to the difference in the refractive index at the interface between the front electrode layer and the semiconductor layer, and thus the quantity of light entering the semiconductor layer for photoelectric effect is decreased. Further, since the reflectance of the incident light at the interface changes depending on the thickness of the front electrode layer, the sensitivity of the color sensor is also influenced by the thickness and accordingly the sensitivity varies greatly among the sensors.